1. Field of the Invention
The invention relates to a method for performing pyrolysis, which is of the type presented in the preamble of the appended claim 1. The invention further relates to pyrolysis apparatus according to the preamble of the appended claim 8.
2. Description of Related Art
Pyrolysis refers to conversion of fuel in inert conditions and high temperature into gaseous form, which during condensation forms oily liquid that comprises different organic compounds. Inert conditions in connection with pyrolysis refer to oxygen-free conditions, wherein combustion of fuel is avoided. Tar distillation is one example of a pyrolysis process known for ages.
In a pyrolysis process fuel is pyrolyzed, the gaseous compounds formed in the reaction are separated from carbonization residue, and they are condensed into pyrolysis oil, which may be used, for exarnple, as fuel or it may be processed further into different chemicals. Production of pyrolysis oil from different bio-based, for example wood based fuels has been studied with the purpose of replacing coal and heavy fuel oil with it. One advantage of pyrolysis oil is its easy transportation in comparison to biomass that is difficult to transport, when taking into account the energy content of fuels.
Examples of development of pyrolysis processes include several patent publications, for example, U.S. Pat. Nos. 4,891,459; 5,728,271; EP 513051 and U.S. Pat. No. 6,814,940. The pyrolysis technique disclosed in these publications is based on the fluidization of solid, for example biomass-based fuel with inert fluidizing gas at approx. 400 to 600° C., possibly in the presence of fluidized bed material. The fuel to be pyrolyzed in a reactor is supplied to the lower part of the reactor, from where it flows upwards with fluidizing gas. The bed material and carbon residue are separated from the gases flowing out of the reactor by cyclones or corresponding separators, as shown, for example, in publication EP 513051 (Ensyn Technologies Inc.).
Publication WO 02/083816 discloses pyrolysis in a fluidized bed reactor, where the aim is to keep the bed dense in order to intensify heat transfer from carrier particles (sand) to fuel particles. The actual reactor is a riser, which is surrounded by carrier particles in return circulation, from which particles the combustible residue formed in them in the pyrolysis reaction is being burned off.
Publication WO 97/06886 (Biomass Technology Group B.V) discloses the circulation of heat treated material inside a reactor by means of a special rotating, vertical, upwards-expanding vessel. One application is the pyrolysis of materials. The publication WO 03/106590 (Biomass Technology Group B.V.) discloses a two-phase process, where the first phase comprises mixing of pyrolyzed particles and hot carrier particles in a mixing chamber, and the second phase comprises the separation of pyrolysis gases from a downward-flowing mixture in a reactor chamber.
The patent FI 117513 (Valtion teknillinen tutkimuskeskus, Technical Research Centre of Finland) discloses a method, where a pyrolyzer is placed in connection with a fluidized bed boiler burning solid fuel, which pyrolyzer utilizes in the pyrolysis the energy content of the hot inert bed material (sand) of the fluidized bed boiler. Different fuel is fed to the pyrolyzer than to the fluidized bed boiler. According to one alternative (FIG. 3) the pyrolyzer is placed as a part of the furnace structure of the fluidized bed boiler and connected to the internal material circulation of bed material.
In the above-described processes, especially when the pyrolyzed material is solid fuel with varying particle size, it is a challenge to have such residence times that the material is pyrolyzed at optimum efficiency. To ensure this, circulation solutions must be developed in the pyrolysis reactor in order to ensure that the material remains in pyrolysis conditions as long as possible.